(1) Field of the Invention
The present invention relates to a sheet discharge processing device for performing discharge processing upon determining in accordance with an instruction whether to discharge a sheet on which an image is formed by an image forming apparatus with the image-bearing surface facing down or up.
(2) Description of the Prior Art
Recently, a digital copying machine has been commercialized as a multi-function machine which operates in the printer mode, the facsimile mode, and the like as well as in the copy mode. In the general copy mode, sheets of paper are sequentially discharged facing up, i.e., with the image-bearing surface facing up. In the printer mode, face-down paper discharge, i.e., paper discharge with each image-bearing surface facing down, is generally performed. More specifically, in the general copy mode, in consideration of the collation of image-formed sheets to be discharged, an automatic document feeder or the like is used to feed originals so as to cause the machine to sequentially form images on sheets from the last page, thereby discharging image-formed sheets in the collated or sorted state. In contrast to this, when the machine operates as a printer, since image data is sequentially transferred from an external device from the first page, image formation is performed from the first page. For this reason, sheets must be discharged without changing their postures or discharged after being reversed.
In order to realize this sheet discharge processing function, for example, Japanese Patent Application Laid-open Hei 5 No. 310357 discloses a device having that function. The arrangement of this device will be briefly described below. As shown in FIG. 21, when a toner image on a photoreceptor 101 is transferred onto a sheet fed from a paper feed tray 100, the sheet passes through a fixing device 102 and is discharged out of the device. A paper discharge processing unit 103 switches the sheet discharge modes in accordance with the copy mode or the printer mode.
When the image forming apparatus is in the copy mode, the sheet is discharged, with the recording surface facing up, onto a first discharge tray 105 through a paper outlet 104 via various convey rollers of a paper discharge processing unit 103. In the printer mode, the convey path is switched to temporarily guide the sheet to a switchback convey path 106 via various convey rollers, and the convey direction is switched by a switching means 107. Thereafter, the sheet is discharged, with the recording surface facing down, onto a second discharge tray 109 through a paper outlet 108.
Referring to FIG. 21, reference numeral 110 denotes an intermediate unit for double-sided image formation which is detachably or integrally mounted on a digital image forming apparatus body 111. When the intermediate unit 110 is mounted on the image forming apparatus body 111, in forming images on the upper and lower surfaces of a sheet, the sheet is conveyed into an intermediate tray 112 via the paper discharge processing unit 103 and the switchback convey path 106, and is conveyed again from the intermediate tray 112 to the transfer position on the photoreceptor 101 on which a toner image is formed.
In order to form images, a laser beam irradiation unit 113 is placed above the photoreceptor 101, and a read unit (scanner) 114 for optically reading an image on an original is placed in the uppermost portion of the apparatus. An image on an original placed on a transparent original table 115 is formed on a CCD 117 as a read element by the scanner 114 through an optical system 116. The image is photoelectrically converted and read by the CCD 117. The semiconductor laser of the laser beam irradiation unit 113 is driven on the basis of the read image data to form an image on the photoreceptor 101.
In discharging sheets on which image are formed by the above arrangement, because of the use of the switchback convey path 106, the sheet convey cycle must be set to a value larger than the length of each sheet in the convey direction so as to prevent the leading and trailing ends of sheets from overlapping. As the image formation speed increases, the convey speed of the switchback convey path 106 needs to be higher than the convey speed in an image formation process. If, however, the convey speed increases, jams and the like may occur frequently.
In order to eliminate such a drawback in the device disclosed in Japanese Patent Application Laid-open Hei 5 No.310357, the device disclosed in Japanese Patent Application Laid-open Sho 60 No.52458 alternately uses two switchback paths to shorten the sheet convey cycle. In this device, as shown in FIG. 22A, a reverse convey device 120 is placed midway along the path extending from an inlet 121 to which a sheet having undergone an image formation process in the image forming apparatus is conveyed to an outlet 122 through which the sheet is discharged. Convey rollers 123 are arranged at the inlet 121, and the fed state of a sheet is detected by a paper feed detection switch 124. Discharge rollers 125 are arranged at the outlet 122.
A switching gate 128 is rotatably placed between the inlet 121 and the outlet 122. The switching gate 128 guides a sheet P to linear switchback paths 126 and 127 extending in the vertical direction. The switching gate 128 has a first guide surface 128a for guiding a sheet to the upper or lower switchback path 126 or 127, a second guide surface 128b for guiding the sheet from the upper or lower switchback path 126 or 127 to the outlet 122, and a straight guide path 128c for causing the sheet to travel straight from the inlet 121 to the outlet 122 without reversing it.
In the above arrangement, when a sheet P having undergone image formation in the image forming apparatus is conveyed to the inlet 121, the sheet P is clamped between the convey rollers 123 to be continuously conveyed. When the leading end of the sheet P is detected by the paper feed detection switch 124, the switching gate 128 is switched first to the state shown in FIG. 22A to guide the sheet P to the upper switchback path 126. After the paper feed detection switch 124 detects that the sheet P is reliably conveyed into the switchback path 126, i.e., after the paper feed detection switch 124 detects the trailing end of the sheet P, and the trailing end of the sheet P is clamped between convey rollers 126a arranged at the switchback path 126, the switching gate 128 is switched to the position in FIG. 22B.
After this operation, the rotating directions of the convey rollers 126a are reversed to guide the sheet P to the discharge rollers 125 at the outlet 122 along the first guide surface 128a. At the same time, the next sheet P having undergone image formation is guided to the second switchback path 127 along the second guide surface 128b of the switching gate 128. At the timing when the trailing end of the sheet P is detected by the paper feed detection switch 124, and the sheet P is clamped between convey rollers 127a at the lower switchback path 127, it is assumed that the sheet P in the switchback path 126 is completely discharged, and the switching gate 128 is rotated through 90.degree. (in the counterclockwise direction).
As described above, after sheets P on which images are formed are conveyed into the upper and lower switchback paths 126 and 127, the sheets P are alternately discharged to the outlet 122. For this reason, even if one sheet P is conveyed into the reverse convey path at the same time another sheet P is conveyed from the reverse convey path to the outlet, the leading and trailing ends of the sheets P dot not overlap, and the sheets P can be reversed/discharged in a shorter cycle.
As described above, with the use of the sheet discharge processing device disclosed in Japanese Patent Application Laid-open Sho 60 No.52458, sheets P can be discharged in a shorter convey cycle. Even if, therefore, the processing speed of an image formation apparatus increases, appropriate discharge processing can be performed.
In the arrangement having two switchback paths, the guide surfaces are arranged to guide the sheet P to the switchback path and also guide the sheet P from the switchback path to the outlet. This arrangement also includes guide plates 129a to 129d and the like in FIG. 22 opposing these guide surfaces to reliably guide the sheet P. However, when the switching gate 128 is to be rotated about a rotating shaft, these guide plates 129 and the like must be retracted. Without the guide plates 129, the switching gate 128 can be rotated without being interfered, and hence the rotation mechanism can be simplified. However, the sheet P is guided unstably, so that a convey failure or a jam of the sheet P may occur frequently when the sheet P is conveyed into the switchback paths 126 and 127 or guided from the switchback paths 126 and 127 to the discharge rollers 125. The sheet P tends to curl in one direction because it passes through the fixing rollers in the process of image formation. For this reason, the sheet P cannot be reliably guided with only the guide surfaces 128a and 128b of the switching gate 128.
In addition, since the switchback paths are formed in a line, a very large wasteful space is required in the vertical direction. Since each switchback path requires a linear distance long enough to store a sheet of the maximum size, a space exceeding twice the maximum size is required. Consequently, the sheet discharge processing device has a large size, and a large wasteful space is formed.
Furthermore, according to the above description, sheets are alternately discharged. In this technique, however, if a sheet jam occurs in the image forming apparatus body, or the image formation cycle becomes unstable, the switching timing of the switching gate greatly shifts, and switching is performed while a sheet is discharged. As a result, the sheet, the switching gage, or the like may be damaged.